You can train a young wisteria into a tree form in the following way: Remove all but one main stem and stake this one securely. Tie the stem to the stake at frequent intervals, using plastic tape to prevent girdling. When the plant has reached the height at which you wish a head to form, pinch or prune out the tip to force branching. Shorten the branches to beef them up and pinch out long streamers. Rub off all buds that form below the head. Replace the stakes and ties as needed. Keep your plant well watered and well-fed during this growing process.